数据库复习之去重数据

1. 查看过滤重复后的数据
思路, group by 分组可以对多个列进行分组, 分组后可以过滤掉重复的数据

SELECT id,un,pw,sex,email FROM USER GROUP BY id

2. 查看重复的数据

SELECT id,un,pw,sex,email,COUNT(1) FROM USER GROUP BY id HAVING COUNT(1)>1 

3. 删除重复的数据留下一条

DELETE FROM USER
WHERE 1=1
AND id NOT IN (SELECT * FROM ( (SELECT MIN(id) ids FROM USER GROUP BY un HAVING COUNT(un)>1) ) b))